The invention relates to a method for determining a toothed rack force for a steering device in a vehicle.
The invention also relates to a steering device in the vehicle. The invention further relates to a control unit for a steering device in a vehicle and to a computer program that can be executed on the control unit.
In modern steering devices, for example in an electric power steering (EPS) system or in what is referred to as a Steer-by-Wire (SbW) steering system, a target steering torque is determined, which is applied to a steering means, for example a steering wheel, in order to support or counteract the force applied by the driver.
EPS steering systems that are currently employed generate an EPS motor torque based on a toothed rack force that is present, so as to provide the driver with the appropriate steering assistance. The toothed rack force is decisively influenced by the current cornering forces. A significant part of the current toothed rack force thus corresponds to transverse acceleration. The toothed rack force, however, is determined not only by lateral forces that occur when traveling in a curve, but rather a variety of other variables of a current driving situation also influence the toothed rack force. One example of this is the roadway condition (unevenness, lane grooves, or the friction coefficient).
Additionally, further functions may be involved in generating the target steering torque so as to achieve a desired and pleasant steering feel for the driver, wherein, on the one hand, undesirable disturbances should not have any influence on the target steering torque, but, on the other hand, safety-relevant information in particular, such as on the current conditions of the roadway surface, should be brought to the attention of the driver via the steering torque.
Compared to hydraulic steering systems, however, such EPS steering systems have disadvantages in terms of the feedback regarding the driving conditions. Given the relatively high inertia of the EPS actuator, in known EPS steering systems, it is of particular note that the behavior at various road friction coefficients or within the physical handling limitations, such as during understeering or oversteering, is only conditionally passed on to the driver via the perceivable steering torque.
It is known to determine the currently present toothed rack force by means of a torque sensor that is disposed on the toothed rack, or by estimation using what is referred to as an observer, which is based on a model of the steering system. Such a method is known, for example, from DE 10332023A1. There, according to a first exemplary embodiment, the steering torque is determined as a function of the lateral force occurring on the steered wheels, and according to a further exemplary embodiment, it is determined as a function of the actual steering torque, so as to determine a steering torque for steering a vehicle. According to the known method, the lateral force is estimated or modeled by means of a sensor, or is based on a model of the steering system of the vehicle, depending on at least one of the variables of transverse acceleration, steering angle and vehicle speed.
The toothed rack force determined by means of the known methods provides the force conditions actually present at the front axle of the vehicle, or at the toothed rack, depending on the quality of the method. If this force is used as a basis for generating what is referred to as a desired steering torque, the driver obtains good feedback on the behavior of the vehicle, or on the conditions of the roadway.
Generating a driver's desired steering torque based on the toothed rack force that is present includes, not only the level that is actually required, but also various disturbances having different characteristics. These are dependent, for example, on the design of the axle(s) and/or the general configuration of the vehicle. Disturbances, here, can be understood to mean a variety of roadway factors such as unevenness, lane grooves or lateral inclination. Additional disturbances can develop as a result of longitudinal dynamics events at the steered front axle. Examples of this include varying lengths of drive shafts in front-wheel drive vehicles, active drive components such as all-wheel drive, or components for variably distributing a driving torque to the front wheels. Another influencing factor that may be perceived as a disturbance can result from the load of the vehicle and, in general, notably resulting from the front axle load. Put in simplified terms, it can be stated that the toothed rack force acting on the steering system increases as the load on the front axle rises. The tires on the wheels additionally influence the toothed rack force or the target steering torque.
It is the object of the present invention to generate a toothed rack force for steering devices in vehicles comprising electric power steering (EPS), which both represents a particularly good basis for generating a driver's desired torque, and is free of disturbances to as great an extent as possible.